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WASHINGTON (CNN) -- New York Mayor Michael Bloomberg said Tuesday he and other city officials weren't about to wait until a security threat materialized before acting.

"We have to protect this city. We can't just sit around with your fingers crossed," said Bloomberg, whose critics say he overreacted to information of a possible terrorist attack on the city's transit system.

"We have to make decisions and show leadership. And that is exactly what we have done. And if given the same situation, we would do exactly the same thing," he said.

Government sources said Tuesday information from an informant in Iraq about a terrorist plot involving New York's subway system was a hoax.

After various investigations, the sources said, officials determined the informant's tip was false.

New York Police Department Commissioner Ray Kelly acknowledged the need to improve communications about intelligence information between federal and local law enforcement officials.

"I think we have got to get together with federal agencies, and they have a responsibility in Washington to speak with one voice to proactively put out information that's going to help localities," he said Tuesday.

Much of the information that led to the heightened security in New York "was gleaned from our initiative and our contacting federal authorities," he said.

"So, yeah, I think that there are lessons here to be learned. I think some congressional committees will, in fact, look into this whole matter and hopefully the system will be improved as a result," he added.

'Specific threat'

According to one official knowledgeable about the investigation, the threat listed this past Friday and Sunday as possible days for an attack.

Friday was three months to the day after four bombers carried out attacks on three London, England, subways and a double-decker bus, killing 52 people and wounding 700. (Full story)

Bloomberg cited information from the FBI about a "specific threat" when he heightened subway security Thursday.

Some intelligence officials downplayed that information, saying it was not credible, and by Monday, law enforcement officials said they could not corroborate any of the informant's claims.

Bloomberg said he was also told that when three men allegedly linked to the supposed New York plot were arrested in Iraq, one of them had shouted, "You are too late to stop us!"

The three men were later interviewed and given lie detector tests that showed they knew nothing about such a plan, according to government sources.

In addition, information alleging that someone involved in a possible New York plot had entered the United States has not been corroborated. "We still do not know if such a person even exists," an official said.